The present invention relates to exhaust gas purifying methods and exhaust gas purifying systems aiming to optimize the start and stop timing of sulfur purging control for the resolution of sulfur poisoning of NOx occlusion reduction type catalysts.
A wide range of research projects and proposals have dealt with the removing of particulate matter (PM) and nitrogen oxides (NOx) from the exhaust gasses generated by automobile internal combustion engines, mounted internal combustion engines, and the like, thus creating cleaner exhaust gasses. With regard to the purifying of NOx from exhaust gasses from automobiles and the like in particular, NOx occlusion reduction type catalysts, three way catalysts, and other NOx purifying catalysts are being used.
These NOx occlusion reduction type catalysts are formed from monolithic catalysts and the like. Furthermore, these NOx occlusion reduction type catalysts are formed by providing a catalyst coat layer to alumina, titanium oxide, and other supporting bodies, and by then supporting NOx occluding materials (or substances) such as platinum and other noble metal catalysts or barium on this catalytic coating layer. NOx can subsequently be purifying from the exhaust gas by occlusion when this gas is in a condition with a high oxygen concentration (i.e., a lean air-fuel ratio state). In addition, when there is little or no oxygen in the exhaust gas (i.e., a rich air-fuel ratio state), the release of NOx to the environment can be prevented by discharge the occluded NOx and reducing the discharged NOx at the same time.
However, sulfur contained in the fuel adheres to the NOx occluding material in the case of this type of NOx occlusion reduction type catalyst, causing the NOx occluded quantity and purifying ratio to drop and resulting in the problem of sulfur poisoning. This sulfur content is adsorbed into the NOx occluding material as Ba2SO4 (barium sulfate) or some other sulfate. Accordingly, CO (carbon monoxide) is replaced with SO2 (sulfur dioxide) in the Ba2SO4 in oxygen-less and high temperature conditions, and the sulfur content is discharged as SO2. It is, therefore, necessary as detailed, for example, in Japanese Patent Laid-Open 2000-145438 to regularly control sulfur purging by setting the oxygen concentration low and by raising the temperature of the exhaust gas to a sulfur purging temperature in excess of the catalyst's regeneration temperature or above.
The specifics of this sulfur-purging control vary with respect to catalyst; however, sulfur purging is generally carried out in a rich environment with the catalyst temperature raised to above 600° C. to 650° C. In order to achieve these conditions in the case of diesel engines, exhaust gas volume reduction is carried out by intake throttling, large-volume EGR, and the like and post injection is carried out. In this way, the air-fuel ratio of exhaust gas flowing into the catalyst becomes rich as its temperature is increased. In addition, the HC (hydrocarbons) and other compounds contained in the exhaust gas are combusted through the catalyst's oxidation action, and the heat produced by this active oxidation reaction causes the temperature of the catalyst to increase further. Accordingly, sulfur purging is accelerated.
As the temperature of the catalyst is increased for this sulfur purging control, there is a danger that heat deterioration will occur if the duration of the process is too long. Accordingly, it is crucial that sulfur purging control be carried out for the minimum necessary time. In addition, if the duration of sulfur purging control after the start of actual sulfur purging is too short, it may not proceed to completion. In such a case, sulfur poisoning will advance and the efficiency of NOx purifying ratio will drop. As a result, accurate judgment of the start time for sulfur purging is extremely important.
Furthermore, sulfur purging control performed for a preset time as determined from experimental results and the like is problematic in that it lacks the ability to compensate for changes in the catalyst over time.
Nevertheless, as SO2 sensors capable of being used in mass production engines do not currently exist, it is not possible to judge the start of sulfur purging or the sulfur purging volume, or to accurately predict the start and end timing of sulfur purging, the period to be required, and the like.
Through experiment and the like, however, the inventors of the present invention have gotten the following information with regard to sulfur purging control.
As shown in FIG. 4, when sulfur purging control is carried out, nitrogen dioxide (NO2) is initially discharged from the NOx occluding material and active oxygen is generated. Following this, the NO2 is reduced by the catalytic action and the oxygen concentration in the exhaust gas is increased. Accordingly, sulfur discharge becomes no longer possible. When the discharge of SO2 from the NOx occluding material and its reduction are close to ending, the oxygen concentration in the exhaust gas drops, and simultaneously, the excess CO begins to slip (i.e., discharge to the flow path downstream of the catalyst). At this time, sulfur purging and the discharge of SO2 also start. Accordingly, by monitoring the oxygen concentration downstream of the catalyst, it is possible to judge the timing of the start of SO2 discharge during sulfur purging based on the change in the oxygen concentration.
In addition, as shown in FIG. 3 through FIG. 5, the sulfur purging volume during this sulfur purging control—in other words, the SO2 discharge volume—increases as the S/V ratio (i.e., flow rate/vessel volume)—in other words, the speed of passage of the exhaust gas—grows larger. Sulfur purging is not possible at an oxygen concentration of 0% or higher, and in this extremely rich condition, the volume of CO discharged from the engine is high. Consequently, as the air-fuel ratio during sulfur purging control decreases—in other words, as it is deeper in rich condition—the sulfur purging volume increases. In addition, the higher the temperature of the catalyst during sulfur purging control is, the higher the sulfur purging volume is. Accordingly, by monitoring SAT ratio, air-fuel ratio, and catalyst temperature as parameters, it is possible to accurately estimate the volume of sulfur purging during sulfur purging control.